Grohe 47 111 thermostat valve/thermoelement cartridge doesn't let cold water through

I have an Integrated Grohmix 34-436, which served us well for the past 10 years:

However few weeks ago, the thermostat valve started misbehaving, not letting enough hot water through. So I bought a replacement and installed it.

To test whether it works or not, I put the lever in the open position and turn on the cold and hot water service stops. Unfortunately only hot water comes out, the cold water is not let through. I tried turning the knob on the thermostat valve to no avail, only the hot water keeps coming out, no matter which position the knob is in.

When I put back in the old thermostat valve, the water is mixed correctly, just not as hot as it should.

Is there a calibration procedure that needs to be done on the valve itself? Besides the front knob, there isn't anything else that could be adjusted.

Is there a correct way to insert the cartridge? There is a longitudinal cut on the cartridge, does it need to be in a certain position when I push it in and before I start to screw it in?

Any thoughts on what could be the problem? At this point I'm thinking the replacement thermostat valve is bad.

I've only needed to call them once, but the tech line at Grohe was both helpful and accurate...I'd go to the source.